1. Field of the Invention
This invention relates to wellbore electric cables, and methods of manufacturing and using such cables. In one aspect, the invention relates to a durable and sealed torque balanced enhanced electric cable used with wellbore devices to analyze geologic formations adjacent a wellbore, methods of manufacturing same, as well as uses of such cables.
2. Description of the Related Art
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation.
Logging tools, which are generally long, pipe-shaped devices, may be lowered into the well to measure such characteristics at different depths along the well. These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline cable connects the logging tool with one or more electrical power sources and data analysis equipment at the earth's surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline cable is spooled out of a truck, over a pulley, and down into the well.
Wireline cables are typically formed from a combination of metallic conductors, insulative material, filler materials, jackets, and metallic armor wires. Commonly, the useful life of a wellbore electric cable is typically limited to only about 6 to 24 months, as the cable may be compromised by exposure to extremely corrosive elements, or little or no maintenance of cable strength members, such as armor wires. A primary factor limiting wireline cable life is armor wire failure, where fluids present in the downhole wellbore environment lead to corrosion and failure of the armor wires.
Armor wires are typically constructed of cold-drawn pearlitic steel coated with zinc for corrosion protection. While zinc protects the steel at moderate temperatures, it is known that corrosion is readily possible at elevated temperatures and certain environmental conditions. Although the cable core may still be functional, it is generally not economically feasible to replace the armor wire, and the entire cable must be discarded. Once corrosive fluids infiltrate into the annular gaps, it is difficult or impossible to completely remove them. Even after the cable is cleaned, the corrosive fluids remain in interstitial spaces damaging the cable. As a result, cable corrosion is essentially a continuous process which may begin with the wireline cable's first trip into the well. Once the armor wire begins to corrode, strength is quickly lost, and the entire cable must be replaced. Armor wires in wellbore electric cables are also associated with several operational problems including torque imbalance between armor wire layers, difficult-to-seal uneven outer profiles, and loose or broken armor wires.
In wells with surface pressures, the electric cable is run through one or several lengths of piping packed with grease, also known as flow tubes, to seal the gas pressure in the well while allowing the wireline to travel in and out of the well. Because the armor wire layers have unfilled annular gaps or interstitial spaces, dangerous gases from the well can migrate into and travel through these gaps upward toward lower pressure. This gas tends to be held in place as the wireline travels through the grease-packed piping. As the wireline goes over the upper sheave at the top of the piping, the armor wires may spread apart, or separate, slightly and the pressurized gas is released, where it becomes a fire or explosion hazard. Further, while the cables with two layers of armor wires are under tension, the inner and outer armor wires, generally cabled at opposite lay angles, rotate slightly in opposite directions, causing torque imbalance problems. To create a torque-balanced cable, inner armor wires would have to be somewhat larger than outer armor wires, but the smaller outer wires would quickly fail due to abrasion and exposure to corrosive fluids. Therefore, larger armor wires are placed at the outside of the wireline cable, which results in torque imbalance.
Armored wellbore cables may also wear due to point-to-point contact between armor wires. Point-to-point contact wear may occur between the inner and outer armor wire layers, or oven side-to-side contact between armor wires in the same layer. While under tension and when cables go over sheaves, radial loading causes point loading between outer and inner armor wires. Point loading between armor wire layers removes the zinc coating and cuts groves in the inner and outer armor wires at the contact points. This causes strength reduction, leads to premature corrosion and may accelerate cable fatigue failure. Also, due to annular gaps or interstitial spaces between the inner armor wires and the cable core, as the wireline cable is under tension the cable core materials tend to creep thus reducing cable diameter and causing linear stretching of the cable as well as premature electrical shorts.
It is commonplace that as wellbore electrical cables are lowered into an unobstructed well, the tool string rotates to relieve torque in the cable. When the tool string becomes stuck in the well (for example, at an obstruction, or at a bend in a deviated well) the cable tension is typically cycled until the cable can continue up or down the hole. This bouncing motion creates rapidly changing tension and torque, which can cause several problems. The sudden changes in tension can cause tension differentials along the cables length, causing the armor wires to “birdcage.” Slack cable can also loop around itself and form a knot in the wireline cable. Also, for wellbore cables, it is a common solution to protect armor wire by “caging.” In caging designs, a polymer jacket is applied over the outer armor wire. A jacket applied directly over a standard outer layer of armor wires, which is essentially a sleeve. This type of design has several problems, such as, when the jacket is damaged, harmful well fluids enter and are trapped between the jacket and the armor wire, causing corrosion, and since damage occurs beneath the jacket, it may go unnoticed until a catastrophic failure.
Also, during wellbore operations, such as logging, in deviated wells, wellbore cables make significant contact with the wellbore surface. The spiraled ridges formed by the cables' armor wire commonly erode a groove in the side of the wellbore, and as pressure inside the well tends to be higher than pressure outside the well, the cable is prone to stick into the formed groove. Further, the action of the cable contacting and moving against the wellbore wall may remove the protective zinc coating from the armor wires, causing corrosion at an increased rate, thereby reducing the cable life.
Thus, a need exists for wellbore electric cables that prevent wellbore gas migration and escape, are torque-resistant with a durable jacket that resist stripping, bulging, cut-through, corrosion, abrasion, avoids the problems of birdcaging, armor wire milking due to high armor, looping and knotting, and are stretch-resistant, crush-resistant as well as being resistant to material creep and differential sticking. An electrical cable that can overcome one or more of the problems detailed above while conducting larger amounts of power with significant data signal transmission capability would be highly desirable, and the need is met at least in part by the following invention.